WIP: Bun support

[markdalgleish]

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[markdalgleish]

Closing this in favour of a different strategy:

• Remove the SuperHeaders class and instead convert @remix-run/headers into a set of utils. This requires updating fetch-router so it uses standard Headers instead of SuperHeaders.
• Make LazyFile/LazyBlob custom classes that don't subclass File/Blob. Instead, you need to call .stream() when you want to use them in a response body. We'll also add LazyFile.toFile() and LazyBlob.toBlob() helpers.
• Split out charset fixes into a standalone PR.